Quinacridones are known compounds which are used as pigments. In practice, stringent requirements are placed on their fastness and color properties. On the industrial scale they are prepared by oxidizing dihydroquinacridones in an alkali medium in the presence of solvents and then dry- or wet-grinding the resulting coarsely crystalline crude pigments, or by ring closure of 2,5-dianilinoterephthalic acid in polyphosphoric acid or polyphosphoric ester and then phase-converting and finishing the resulting finely divided crude pigments with organic solvents.
The preparation of quinacridone mixed crystal pigments is described in the following patent documents.
U.S. Pat. No. 4,099,980 describes the preparation of quinacridone mixed crystal pigments, consisting of unsubstituted quinacridone and 4,11-dichloroquinacridone, which are, however, in the y phase of the unsubstituted quinacridone.
U.S. Pat. No. 3,160,510 describes the preparation of quinacridone mixed crystal pigments by dry-milling the crude pigment mixtures with salt and then solvent-treating the isolated ground materials or by precipitating the pigment mixtures with sulfuric acid and then solvent-treating the dried finely divided crude pigments.